The pivotal event in vertebrate blood cell differentiation is the developmental decision wherein a stem cell either undergoes perpetual self renewal or initiates terminal differentiation into any of the variety of blood cell lineages. Conditions which influence this developmental decision are not well understood, primarily because of our inability to isolate hematopoietic progenitor cells in sufficient quantity for biochemical or genetic analysis. We propose to isolate pluripotent and bipotent hematopoietic progenitor cells using a genetically altered, conditionally transforming retrovirus. Transformed cells having the desired properties (those cells which can give rise to multiple blood lineages) will be identified by the ability to express a variety of cell surface markers unique to mature erythroid, myeloid and lymphoid cells, and by their ability to give rise to mixed colonies in vitro when these precursor cells are shifted to conditions which inactivate the transforming gene(s). Transformed progenitor cells will then be used to analyze the expression of a variety of tissue-specific genes in attempts to address fundamental questions about the establishment of cell lineage and the nature of developmentally specific gene control.